388 Harper.—Sexual Reproduction in Pyronema 
doubtful relationship to the main series may bring further 
evidence as to its phylogeny. 
If we turn now to problems of relationship inside the group 
itself, and compare the sexual apparatus as we find it in 
Pyronema and Sphaerotheca , we are confronted by some 
marked differences in detail. A conjugation-tube or trichogyne 
is entirely wanting in Sphaerotheca. A further difference lies 
in the fact that in Pyronema the oogonium functions without 
further growth as an ascogonium. All the fertilized nuclei 
pass into ascogenous hyphae and may reach the asci, while 
in Sphaerotheca the single fertilized egg develops a row of 
cells of which only one becomes an ascus, while the others 
undergo degeneration. There is nothing in Pyronema to 
correspond to this growth of an ascogonium from the fertilized 
egg, and in this respect it seems to represent a simpler con¬ 
dition than Sphaerotheca. Also the fact that the entire pro¬ 
toplasmic mass of the oogonium, except in the case of 
unfertilized nuclei noted above, passes out into the ascogenous 
hyphae seems perhaps to indicate a simpler condition. It is 
perhaps analogous to the setting free of the egg-protoplasm 
from the oogonial cell wall which we find in those oogonia 
which produce regularly one or more free oospheres in their 
interior. 
It is possible again that the ascogonium in Sphaerotheca 
and Erysiphe is in reality comparable to a single ascogenous 
hypha in Pyronema , in which case the two types could be 
conceived as more nearly related. Such comparisons, how¬ 
ever, may very well be deceptive, since it has been shown 
many times that resemblance in form may be found where no 
genetic relationship between the parts or organisms compared 
can be assumed. 
So far, however, as the trichogyne is concerned, we 
can readily imagine the development of the type in Sphaero¬ 
theca into that in Pyronema. The oogonium would need 
only to develop a conjugating beak such as is seen in 
Coleochaete , and this could be readily conceived as gaining 
final differentiation as a cell by the putting in of the partition- 
